DUFFKING wrote:
I dunno. There's been two amy ponds in the Tardis before, actually, albeit only in the Comic relief shorts.

Amy actually touched her past self in The Big Bang (when Amelia let Amy out of The Pandorica). Granted, it was from a different timeline but under the ''rules'' of Doctor Who that should have caused a temporal short-out.

The excuse was the universe was fairly screwed up at that point so he could get away with things he couldn't otherwise ordinarily do.

But maybe the point was that the Tardis couldn't contain two Amys from two different timestreams (this episode) but it can manage with two Amys from the same timestream (Comic Relief).